1. Field of the Invention
The present invention relates to an ex-core nuclear instrumentation system that monitors neutron flux outside a reactor vessel and, more particularly, relates to a detector signal processing circuit (I/E amplifier) to be located in an ex-core nuclear instrumentation panel that constitutes the ex-core nuclear instrumentation system.
2. Description of the Related Art
An ex-core nuclear instrumentation system continuously monitors neutron flux outside a reactor vessel of a pressurized water reactor (PWR) and accordingly the state of the reactor at start and in operation is monitored; and when an abnormality is detected in the condition of the neutron flux, the ex-core nuclear instrumentation system outputs an alarm signal and a signal for emergency shutdown of the reactor and accordingly the reactor is protected. The ex-core nuclear instrumentation system mainly includes a neutron detector that measures the neutron flux and converts the same into a current value and an ex-core nuclear instrumentation panel that performs arithmetic processing of the converted current value to convert into the signals.
Generally, a neutron measurement range of the ex-core nuclear instrumentation system is divided into a neutron source range, an intermediate range, and an output range (operation range) depending on the level of neutron flux from a stopped state to output operation of the reactor. The structure and function of the neutron detector and the ex-core nuclear instrumentation panel is different for each range, and a detector signal processing circuit (I/E amplifier, that is, current/voltage amplifier) is used for arithmetic processing of the output range. The present invention relates to the detector signal processing circuit in the output range.
FIG. 5 is a configuration diagram showing a general configuration of an ex-core nuclear instrumentation system in an output range. An ex-core nuclear instrumentation system 14 includes a neutron detector 3 and an ex-core nuclear instrumentation panel 1. The neutron detector 3 is located in a plural number around outside a reactor vessel 16 to be provided inside a reactor containment vessel 15. The neutron detector 3 measures neutron flux leaked from the reactor vessel 16 and converts the same into a current value. The current value is inputted to a detector signal processing circuit 8 of the ex-core nuclear instrumentation panel 1 and is converted into an output voltage corresponding to a reactor power level. Then, the output voltage of the detector signal processing circuit 8 is inputted to a signal processing card 11. The signal processing card 11 performs analog/digital (A/D) conversion and engineering value conversion, and outputs various signals to an operation panel 12 and an input and output card 13 in a reactor protection based system 2.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2003-8375 (FIG. 1, FIG. 2, and Paragraph Number [0015])
A relatively large number of neutrons are leaked from the conventional PWR reactor vessel, and the neutron detector current value is relatively large (for example, 100 μA to 3 mA) in the detector signal processing circuit 8 in the ex-core nuclear instrumentation panel 1 corresponding to the output range during the operation of the reactor. In an advanced PWR (APWR) that is improved from the conventional PWR, a small number of neutrons are leaked outside the reactor vessel by an improvement in performance of a reflector that makes the neutrons reflect in the reactor vessel; and therefore, a value (for example, 1 μA to 30 μA) which is minuter than the conventional neutron detector current value needs to be measured.
For this reason, a problem exists in that, even when a neutron detector current is amplified by the detector signal processing circuit 8 corresponding to a conventional neutron detection current processing range, an output voltage level (for example, 3.3 V being amplified) cannot be obtained. The output voltage level corresponds to a reactor power level of 100% which is capable of performing arithmetic processing after the signal processing card. Furthermore, there needs a circuit configuration which achieves a high gain capable of satisfying an output voltage level corresponding to a neutron detector current processing range (1 μA to 3 mA in the case of the above example) of ex-core nuclear instrumentation that corresponds to both the conventional PWR and APWR.
Further, there needs a circuit configuration which is for obtaining a detector signal processing circuit (I/E amplifier) capable of achieving a high gain and maintaining the accuracy of a measured value by amplifying a minute current by an I/E amplifier with a high gain. Incidentally, a document shown in Patent Document 1 discloses as a variable gain amplifier.
The present invention has been made to solve the problem described above, and an object of the present invention is to provide an ex-core nuclear instrumentation system in which, even when a neutron detector current is minute, a voltage level corresponding to a reactor power level can be outputted, the correct condition of neutron flux can be obtained, and a highly accurate measured value can be obtained.